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Breast cancer is a malignant (cancer) tumor that starts in the cells of the breast. It is found mostly in women, but men can get breast cancer, too. This information is only about breast cancer in women. You can learn more about breast cancer in men in Breast Cancer in Men.

The normal breast

To understand breast cancer, it helps to know something about the normal parts of the breasts, as shown in the picture below.

A woman's breast is made up of glands that can make breast milk (lobules), small tubes that carry milk from the lobules to the nipple (ducts), fatty and connective tissue, blood vessels, and lymph vessels. Most breast cancers begin in the cells that line the ducts. Fewer breast cancers start in the cells lining the lobules. Cancers can also start in cells of the other tissues in the breast. These are called sarcomas and lymphomas and are not really thought of as breast cancers.

The lymph system of the breast

The lymph system is one of the main ways breast cancer spreads. Normally, lymph nodes are small, bean-shaped tissues that contain a certain kind of immune system cell (cells that fight infections). Lymph nodes are connected by vessels (like small veins) that carry a clear fluid called lymph instead of blood.

Most of the lymph vessels of the breast drain into:

  • Lymph nodes under the arm (axillary nodes).
  • Lymph nodes around the collar bone (supraclavicular and infraclavicular lymph nodes)
  • Lymph nodes inside the chest near the breast bone (internal mammary lymph nodes)

Breast cancer cells can travel in lymphatic vessels and begin to grow in lymph nodes. If cancer cells spread to lymph nodes, there is a greater chance that the cells have also spread to other places in the body. The chance goes up the more lymph nodes with cancer there are. Since cancer in lymph nodes doesn’t always cause the nodes to get larger, doctors often remove one or more lymph nodes to check for cancer spread. Your treatment plan will depend on whether or not cancer is found in the lymph nodes.

Breast lumps that are not cancer (benign breast lumps)

Most breast lumps are not cancer – they are benign. Benign breast tumors are abnormal growths, but they do not spread outside of the breast and they are not life threatening. But some benign breast lumps can increase a woman's risk of getting breast cancer.

For more on benign breast changes, see Non-cancerous Breast Conditions.

Cancer terms

Your doctor may use terms you haven’t heard before to talk about breast cancer. Here are some of the key words you might hear:

Carcinoma: a cancer that begins in the lining layer of organs such as the breast.

Adenocarcinoma: a type of carcinoma that starts in gland tissue (tissue that makes and secretes a substance). The ducts and lobules of the breast are gland tissues because they make breast milk, so cancers starting in these areas are often called adenocarcinomas. Nearly all breast cancers are adenocarcinomas.

Sarcoma: a cancer that starts from connective tissues such as muscle tissue, fat tissue or blood vessels. Sarcomas of the breast are rare and are not covered here.

Types of breast cancers

There are many types of breast cancer, but some of them are very rare. Sometimes a breast tumor can be a mix of these types.

Ductal carcinoma in situ (DCIS)

DCIS means that abnormal cells start in the cells lining the ducts without growing (invading) through the walls of the ducts into the tissue of the breast. Because the cells haven’t invaded, DCIS is also sometimes called a non-invasive breast cancer. Since the cells haven’t grown through the duct wall, they cannot spread to lymph nodes or other organs. But sometimes DCIS can go on to become an invasive cancer. That is why it is sometimes called a pre-cancer. Mammograms find many cases of DCIS.

Invasive (or infiltrating) ductal carcinoma (IDC)

This is the most common breast cancer. It starts in the cells lining a duct, and then the abnormal cells break through the wall of the duct and grow into (invade) the tissue of the breast. From there, the cancer cells can spread to nearby lymph nodes or other parts of the body.

Invasive (infiltrating) lobular carcinoma (ILC)

This cancer starts in the cells lining the milk glands (the lobules). The cells grow through the wall of the lobules and then can spread to nearby lymph nodes or other parts of the body.

Inflammatory breast cancer (IBC)

This is a rare type of invasive breast cancer. Often, there is no single lump or tumor. Instead, IBC makes the skin of the breast look red and feel warm. It also may make the skin look thick and pitted, something like an orange peel. The breast may get bigger, hard, tender, or itchy.

In its early stages, inflammatory breast cancer is often mistaken for infection. Because there is no defined lump, it may not show up on a mammogram, but other tests may be helpful. It has a higher chance of spreading and a worse outlook than other types of breast cancer. For more details, see Inflammatory Breast Cancer.

There are also many other less common types of breast cancer. More details about these can be found in the detailed guide Breast Cancer.

Last Medical Review: 09/09/2014
Last Revised: 01/14/2016